The field of the present invention is methods for controlling automatic transmissions of vehicles.
A known method for controlling continuously variable transmissions is described, for example, in Japanese Patent Application Publication No. 57-161346, best explained with reference to FIG. 1 herein. First, all of the sensors and instruments within the transmission control system are initialized, step (s-1). Subsequently, the rotational speed of the engine (Ne), the speed of the vehicle (V), and the transmission ratio (R) are read by a control unit, step (s-2). In making reference to a transmission ratio herein, the transmission ratio is understood to denote a quotient of rotational speed of the input shaft into the transmission divided by the rotational speed of the output shaft from the transmission.
An objective transmission ratio (Rm), which is the desired transmission ratio to which the transmission is to adjust, is calculated as a function of only the rotational speed (Ne) of the engine, step (s-3). Generally, the objective transmission ratio (Rm) is higher than an actual ratio (R) when the rotational speed (Ne) of the engine is lower than a set value; and (Rm) is set lower when (Ne) is high.
The actual transmission ratio (R) is compared with the objective transmission ratio (Rm), step (s-4). If the objective and actual ratios are the same, control proceeds to step (s-5). If not, control proceeds to step (s-6). In step (s-5), the transmission ratio (R) is maintained at its present value and the control returns to step (s-2). If the objective and actual ratios are not the same, a comparison is made as to which is larger in step (s-6). If the objective transmission ratio (Rm) is smaller, control proceeds to step (s-7) where the actual ratio (R) is decreased. If the actual ratio (R) is smaller than the objective ratio (Rm), control proceeds to step (s-8) where the actual ratio (R) is increased. Following either of step (s-7) or step (s-8), control returns to step (s-2).
The following problems reside in the foregoing conventional method of procedure for controlling transmission ratios of an automatic transmission system.
(1) In conventional systems, the transmission is often set at a high ratio when the vehicle starts moving. This makes it difficult to shove start the vehicle.
Shove starting a vehicle is contemplated here to occur when the engine of the vehicle is caused to start using the vehicle inertia. There are some situations where the vehicle is often shove started. For example, a motorcycle is often shove started when used in off-road racing. When a vehicle engine stops unintentionally while the vehicle is moving, restarting may be achieved by the vehicle inertia.
When shove starting a vehicle equipped with a manual transmission by pushing the vehicle, the transmission ratio is most conveniently set to the second or third gear position so as to give sufficient torque and rotational speed to the engine. However, with a conventional automatic transmission, the transmission ratio is set to a high ratio, corresponding to a low gear position when starting. Thus, when pushing the vehicle by hand for a shove start, the reaction force resisting the pushing makes it difficult to gain sufficient speed to start the engine.
(2) According to conventional methods, the rotational speed of the engine is allowed to vary beyond the most powerful speed range. Thus, greatest available power cannot be realized throughout the driving range.
In vehicles equipped with conventional manual transmissions and clutches, the clutch may be slipped with the transmission in low gear to achieve rapid acceleration beginning from a low speed. According to conventional control methods with variable transmissions, on the other hand, if clutch slippage is used and engine speed becomes high, the transmission ratio (R) is reduced, adversely affecting acceleration.
(3) Under conditions when the driving wheel looses traction due to spinning or locking, conventional control methods for continuously variable transmissions adjust to the wheel speed in the spinning or locked condition. Consequently, the ratio is adjusted to an inappropriate level such that engine braking is inappropriately applied or acceleration is compromised when the wheel again grips the driving surface.
Once a driving wheel slips relative to the ground or driving surface because of an excessively high driving force being transmitted to the driving wheel, rotational speed of the engine (Ne) rises inappropriately and the transmission ratio (R) is shifted to a low ratio. Therefore, when the driving wheel again grips the driving surface, the rotational speed (Ne) of the engine is brought down, resulting in power loss. On the other hand, if wheel slippage is caused by excessive braking force such that the driving wheel may lock or approach the locked condition, the engine speed is lowered and the control circuit accommodates that condition by readjusting the ratio to raise the engine speed. Once in this condition, the vehicle is unintentionally slowed down when the driving wheel regrips the surface.
(4) Similarly, while jumping a motorcycle or other vehicle, the rotational speed of the driving wheel becomes mismatched with the actual speed of the vehicle. Again, conventional automatic systems inappropriately adjust to the wheel speed rather than the vehicle speed.
When a vehicle jumps and the driving wheel comes off the ground, the driving wheel looses traction and the rotational speed of the engine momentarily increases. According to conventional control methods for continuously variable transmissions, the transmission ratio is then reduced. The vehicle then returns to the ground and the wheel slows to match the vehicle speed. With the adjusted ratio, the driving wheel does not regrip the ground quickly and acceleration is compromised. Further, the rotational speed of the engine momentarily slows when the wheel regrips the surface resulting in a further compromise to acceleration. In a condition where the throttle is closed while the vehicle is in the jump, the rotational speed of the engine decreases and the transmission ratio becomes higher. Under this circumstance, when the vehicle regrips the driving surface, braking may be experienced In both cases, performance is lost.
(5) Such conventional control systems for continuously variable transmissions also are inconvenient because the transmission is often not coupled with the engine by a clutch controlled by the operator and the transmission ratio also cannot be controlled by the operator. Other driving conditions not mentioned above can also adversely affect performance.